Read an EPRT endpoint without IPv4 assumptions

The delimiter separates fields; colons inside an IPv6 address stay inside its address field.

• EPRT MUST be followed by one ASCII space, then the chosen delimiter and the three fields, each followed by that delimiter: EPRT <d><net-prt><d><net-addr><d><tcp-port><d>. FTP commands end with CRLF (RFC 959 §4.1.3).
• The delimiter MUST be ASCII 33–126 inclusive. The vertical bar | is RECOMMENDED unless it conflicts with a character needed in the network address. A delimiter must still permit the chosen fields to be read unambiguously.
• The network protocol field MUST be an assigned address-family number. The shown families are 1 for IPv4 dotted decimal and 2 for IPv6 textual addresses. IPv6 colons are not PORT-style comma-separated IPv4 octets.
• The port is the decimal TCP listening-port number. A well-formed supported endpoint is not proof of authorization, identity or reachability. The malformed example chooses standard negative reply 501; RFC 2428 also permits 500 for relevant errors.

Documentation addresses and chosen listening ports. Each chapter is a separate bounded example; FTP authentication, authorization, file commands and transfer replies are not a complete FTP implementation here. EPRT/EPSV provide no encryption or guaranteed reachability.